The present invention relates broadly to the field of atomic frequency standards and is more particularly directed to atomic frequency standards of the type wherein atomic resonance of source atoms is achieved by optical pumping, that is to say, by irradiating the source atoms contained in a sealed chamber with. light energy.
In one type of known optically pumped atomic frequency standard rubidium 87 atoms, in admixture with one or more noble buffer gases, are contained within a glass resonance cell situated in a microwave cavity. These rubidium atoms are excited by exposing them to the sufficiently intense radiation of an appropriately filtered rubidium 87 spectral lamp. The net effect of this filtered light radiation exposure is to cause the rubidium 87 atoms to populate the ground hyperfine level F=2 at the expense of the F=1 level, a phenomenon of atomic physics termed "population inversion." The excited atoms diffuse slowly through the noble gas atoms and during this time they interact with the microwave cavity field. When the microwave frequency corresponds to the rubidium 87 magnetic hyperfine resonance there is a change in the absorption coefficient for the optical pumping radiation and this change is detected by a photodetector stationed to intercept light transmitted through the cell.
With the advent of solid state laser technology substantial efforts have also been directed, with relative success, towards replacement of the spectral lamp optical pump described above with. a solid state diode laser emitting at the proper wavelength and at sufficient intensity, either D.sub.1 or D.sub.2, but tuned to one of the hyperfine ground states. Where cesium is the source resonance atom to be utilized only a laser can be utilized as the optical pump since no isotopic hyperfine filtering of a cesium spectral lamp is possible, cesium having no other stable isotopes. As in the case of the spectral lamp pumped type of atomic frequency standard described above, however, the resonance cell containing the alkali metal resonance source atoms in admixture with. one or more buffer gases, is contained within a microwave cavity tuned to the transition between which the population inversion has been created. Microwave energy is fed to the cavity and its effect on the resonance atoms, is to alter the population of the two levels of the ground state and, consequently, the optical transmission through the cell. The ground state hyperfine resonance signal is thus detected on the transmitted light and is used to lock the frequency of the microwave source used to feed the cavity.
Both of the above-described types of optically pumped atomic frequency standards, whether the exciting light source is a spectral lamp or a laser, require the presence of a tuned microwave cavity as an essential element of the standard. This requirement for a microwave cavity not only contributes to the overall mechanical and operational complexity of an atomic frequency standard based thereon and on the phenomenon of population inversion, but is also limiting of the availability of substantial reductions in the dimensions of the device. It is also of significance that both of the above-described types of atomic frequency standards are bottomed upon intensity optical pumping because, whether a spectral lamp or a laser is employed, they both rely on the intensity of the light source to prepare the resonance source atoms, rather than upon its coherence.
Even more recently, optically pumped atomic frequency standards have been developed wherein the coherence property of a laser, rather than its intensity, is utilized to prepare the energy states of the resonance source atoms and in which the quantum physics phenomenon of Coherent Population Trapping (hereinafter CPT) take place wherein population inversion is avoided and the ground state energy populations of the resonance source atoms remain unaltered. The drawing hereof depicts a generalized scheme in which the CPT frequency standard is achieved. Referring to said drawing, there is provided a sealed, optically transparent resonance cell containing the alkali metal resonance source atoms in admixture with one or more buffer gases. The beam of a laser optical pump of appropriate wavelength for the particular alkali metal utilized as the atomic resonance source is linearly and circularly polarized and then directed into said resonance cell. Where a single laser light source is employed as the optical pump it is modulated over a frequency range, including a subharmonic of the hyperfine ground state 0--0 transition frequency of the alkali metal atoms defining the atomic resonance source, thereby causing the laser to emit as sidebands two radiation fields whose frequency difference is equal to the hyperfine frequency of the atomic resonance source atoms. Where two laser sources are employed, said lasers a phase-locked to one another with a frequency separation equal to the hyperfine frequency of the atomic resonance source, thereby to also establish two radiation fields of the type described above in respect of a single laser optical pump operated with two sidebands. In either case, therefore, the alkali metal resonance source atoms within the resonance cell are submitted to these radiation fields and resonance of said atoms takes place wherein a strong coherence of the ground state occurs at the hyperfine frequency and wherein transitions to the excited P state are inhibited. Thus, at resonance, all alkali metal resonance source atoms within the resonance cell are trapped in the ground state, no transitions take place from the ground state to the excited P state and no energy is absorbed from the laser radiation due to such. transitions. The resonance phenomenon is signalled by: (a) a sharp increase in the intensity of the laser radiation transmitted through the cell along the laser beam axis and/or (b) by a sharp decrease in the intensity of fluorescence transmitted from the cell normal to the laser beam axis. Thus, either or both of these CPT resonance phenomena are detectable by stationing photodetector means: (i) to receive and detect the intensity of the laser light source beam transmitted through the resonance cell along the beam axis and/or (ii) to receive and detect the fluorescent light generated within the resonance cell normal to the beam axis. Atomic frequency standards based upon the general CPT technology outlined above hold much promise in permitting substantial reductions in size of atomic frequency standards from those of the prior art, due in large measure, to the absence, in CPT base standards, of the need for a relatively bulky and often operationally troublesome microwave cavity to surround the resonance cell.
As will be appreciated by those of skill in the art, in any optically pumped atomic frequency standard, and particularly those of the types described generally hereinabove, it is important that the light intensity of the excitation or pumping light radiation entering the resonance cell be closely controlled and maintained as constant as possible throughout the service life of the standard. In the case of the last-mentioned CPT based atomic frequency standards, for instance, undue variations in the intensity of the solid state laser beam entering the resonance cell can adversely affect both the long term stability of the signal-to-noise ratio of the standard over its service life as well as the short term A.M. noise experienced. These fluctuations in solid state laser pumping light intensity, if not adequately controlled, can also adversely affect detected atomic frequency signal level and broaden signal width. Where A.M. noise is substantially reduced or eliminated from the pumping radiation, short term frequency stability (.sigma.(.pi.)) of the standard is substantially improved. Similar considerations attend other types of optically pumped atomic frequency standards. In accordance with the present invention, therefore, the foregoing objects have been, at least in large measure, fully achieved.